Glucose & insulin
Fasting insulin
Anabolic hormone; the earliest detectable signal of insulin resistance.
Sample
Serum, fasting
Fasting
≥10 hours
Clinical reference range
2 to 25 µIU/mL
Lab range
Conv: 2–25 µIU/mLFunc: 2–6 µIU/mL
Biology
What it measures
Secreted by pancreatic β-cells in response to glucose, GLP-1, and amino acids. Suppresses hepatic gluconeogenesis and lipolysis; promotes glucose uptake.
Clinical use
Why we order it
Detect insulin resistance years before glucose dysregulation; differentiate type 1 from type 2 (low C-peptide in T1).
Lens · Clinical
Interpretation by lens
Clinical interpretation
Insulin assays vary widely between platforms — interpret trends within one assay rather than absolute values across labs.
Functional interpretation
Fasting insulin >10 µIU/mL with normal glucose is early metabolic syndrome; intervene with diet, exercise, and consider metformin in selected patients.
Research note
C-peptide more accurately reflects endogenous insulin production because it has no first-pass hepatic clearance.
Differential
Causes of abnormal values
Causes of HIGH
- ↑Insulin resistance / metabolic syndrome
- ↑Cushing's
- ↑Acromegaly
- ↑Insulinoma
- ↑Exogenous insulin (with low C-peptide)
Causes of LOW
- ↓Type 1 diabetes
- ↓Late-stage type 2 (β-cell exhaustion)
- ↓Fasting / starvation
Evidence-graded claims
What the data says
B
Fasting insulin detects insulin resistance years before fasting glucose rises
Reproducible in cohorts; assay variability limits clinical use.
B
HOMA-IR is a clinically useful insulin-resistance metric
Validated against clamp; population-specific cutoffs.
Related